The present invention relates to a friction material which is made by sintering and which is intended for use on contacting surfaces of couplings or brakes, the friction material containing powdery and/or granular metallic and non-metallic components, at least one of which is a non-ferrous heavy metal.
Friction materials of this kind, which are made by sintering, are known and are used primarily in the construction of motor vehicles, but are used also in general machine construction, in various gears and in braking devices. The known friction materials are made of a copper-based sintered powder. Sintered powders to which are added, in addition to copper as the main component, other metals such as tin, zinc, etc., are known from JP-A-63 109 131, 62 260 030, 61 207 549, 61 067 737, 60 116 751, 58 151 444 and 58 079 073. The metallic components are added in elementary form, i.e., as copper, tin, zinc, etc. Since the sintering does not take place in the molten phase but rather in the "dough-like state," copper is contained in a free form in the sintered friction linings. The sintering temperature for metallic friction linings is about 800.degree. C. whereas the melting temperature of the main component, copper, is 1083.degree. C.
The advanced development in the field of gear mechanisms requires that ever more power is concentrated in less space, a feature that in turn requires that the individual subassemblies be adapted to this increased load. The friction materials and the gear oils used for lubrication are also affected by this. To meet the high requirements, high-alloy oils with special oil additives must be used. The improvement in the gear oil properties has an advantageous effect on the loading capacity, reduction of wear, lifespan, frictional behavior, aging of the oil, noise absorption, reduced use of fuel due to the improved efficiency of the gears, etc. However, it also brings functional drawbacks with respect to the friction elements installed into the gears, in particular when their rubbing surfaces are provided with the prior art sintered friction materials.
Due to the effect of the high-alloy oils on the prior art copper-based friction materials, reaction layers form on the friction materials, adversely affecting their frictional behavior. The drop in the coefficient of friction caused by the layer formation leads to malfunctions. In synchronization devices of motor vehicle gears, the losses of the moment of friction lead, e.g., to a deterioration in the switching quality in connection with unpleasant noises. The chemical reaction of the oil additives with the copper of the sintered friction materials also leads to the decomposition of the lining structure, a feature that causes a significant increase in the wear index. The bonding zones of the metallic lining particles are decomposed on the grain boundaries of the porous lining structure, and the inner lining bond collapses with increasing operating time.
The present invention is based on a desire to improve the prior art friction materials, made by sintering, and their resistance to aggressive oils, and to find a sintered friction material which is provided through powder metallurgy and which exhibits good stability to the effect of aggressively acting oil additives and thus prevents destructive chemical reactions which have a negative impact on the frictional and wear properties of the known friction materials.